Hydrogen is an appealing energy carrier for clean energy use. However, storage of hydrogen is still the main bottleneck for the realization of an energy economy based on hydrogen. Many materials with outstanding properties have been synthesized with the aim to store enough amount of hydrogen under ambient conditions. Such efforts need guidance from material science, which includes predictive theoretical tools. Carbon nanotubes were considered as promising candidates for hydrogen storage applications, but later on it was found to be unable to store enough amounts of hydrogen under ambient conditions. New arrangements of carbon nanotubes were constructed and hydrogen sorption properties were investigated using state-of-the-art simulation methods. The simulations indicate outstanding total hydrogen uptake (up to 19.0 wt.% at 77 K and 5.52wt.% at 300 K), which makes these materials excellent candidates for storage applications. This reopens the carbon route to superior materials for a hydrogen-based economy. Zeolite imidazolate frameworks are subclass of MOFs with an exceptional chemical and thermal stability. The hydrogen adsorption in ZIFs was investigated as a function of network geometry and organic linker exchange. Ab initio calculations performed at the MP2 level to obtain correct interaction energies between hydrogen molecules and the ZIF framework. Subsequently, GCMC simulations are carried out to obtain the hydrogen uptake of ZIFs at different thermodynamic conditions. The best of these materials (ZIF-8) is found to be able to store up to 5 wt.% at 77 K and high pressure. We expected possible improvement of hydrogen capacity of ZIFs by substituting the metal atom (Zn 2+) in the structure by lighter elements such as B or Li. Therefore, we investigated the energy landscape of LiB(IM)4 polymorphs in detail and analyzed their hydrogen storage capacities. The structure with the fau topology was shown to be one of the best materials for hydrogen storage. Its total hydrogen uptake at 77 K and 100 bar amounts to 7.8 wt.% comparable to the total uptake reported of MOF-177 (10 wt.%), which is a benchmark material for high pressure and low temperature H2 adsorption. Covalent organic frameworks are new class of nanoporous materials constructed solely from light elements (C, H, B, and O). The number of adsorption sites as well as the strength of adsorption are essential prerequisites for hydrogen storage in porous materials because they determine the storage capacity and the operational conditions. Currently, to the best of our knowledge, no experimental data are available on the position of preferential H2 adsorption sites in COFs. Molecular dynamics simulations were applied to determine the position of preferential hydrogen sites in COFs. Our results demonstrate that H2 molecule adsorbed at low temperature in seven different adsorption sites in COFs. The calculated adsorption energies are about 3 kJ/mol, comparable to that found for MOF systems. The gravimetric uptake for COF-108 reached 4.17 wt.% at room temperature and 100 bar, which makes this class of materials promising for hydrogen storage applications.

The crystal growth of nanoporous materials is different from most other classes of material in that their framework architectures contain periodic arrangement of pores or voids in which there is no direct bonding between adjacent units of the framework. This poses a variety of questions as to how such parts of framework develop during the crystallization process, atomistically and energetically. Here we use the nanoporous metal-organic framework, ZIF-8 as a prototypical material to obtain a basic understanding of the growth of a nanoporous material. The crystals of ZIF-8 produced in the N,N-dimethylformamide solvent [ZIF-8(DMF)] and methanol-co-N,N-dimethylformamide solvent [ZIF-8(MeOH)] are both rhombic dodecahedron in shape with a much smaller crystal size in the latter. In the study of the kinetics of ZIF-8(DMF) crystallization, we get a good agreement in the values of activation energies using both Avrami-Erofe’ev-Hancock-Sharp and Gualtieri’s models, i.e. about 120 kJ mol-1 for nucleation, and 95 kJ mol-1 for crystal growth process. The study of kinetics of ZIF-8 surface growth, by in situ AFM, with ZIF-8(DMF) as seed crystal that are grown in the methanolic growth solution we see faster rate in the <100> directions than the <110> directions, with the most probable activation energy of about 80 kJ mol-1 in both directions. This is the first example of in situ AFM being used to obtain activation energy for a surface growth in MOF. We also reveal here that growth process of ZIF-8 occurs through the nucleation and spreading of successive metastable unenclosed sub-steps to eventually form stable terrace steps of the enclosed framework structure in which this process is reliant on the presence of nonframework species to connect the framework species that have voids between them. The experiments also enable identification of some of the fundamental units in the growth process and the stable crystal surface plane. Further, the spreading of terraces at high supersaturation condition (early state) is fairly isotropic as is seen through the formation of almost-rounded terraces on the surface of ZIF-8. The growth direction becomes clear as the supersaturation condition nears to equilibrium (later stage) by the formation of rhombohedral terraces with pointy ends growing along the <100>, and <110> directions and straight edges growing perpendicular to the <111> direction. Formation of this rhombohedral morphology is explained by a coarse grain approach similar to that used in the Kossel model by making assumptions that the sodalite cage is the growth unit and attachment of one sodalite cage in each growth direction is the rate determining step for the formation of a new row of sodalite cages in each direction. Finally, based on the profiles of growth spirals formed from screw dislocations on the ZIF-8 surface obtained from the ex situ AFM images and ICE theory, plausible screw dislocations with Burgers’ vector of 1/2 <111> and <100>, but not <110>, are deduced. Some of the findings in this work will be applicable to numerous nanoporous materials, and the work in general will support efforts to synthesize and design new framework materials and to control the crystal properties of these materials.

Zeolitic imidazolate frameworks (ZIFs) is a new class of metal-organic frameworks (MOFs) with zeolite-like properties such as permanent porosity, uniform pore size, and exceptional thermal and chemical stability. Until recently, ZIF materials have been mostly synthesized by solvothermal method. In this thesis, further analysis to tune the size and morphology of ZIF-8 is done upon our group’s recent success in preparing ZIF-8 crystals in pure aqueous solutions. Compositional parameters (molar ratio of 2-methylimidazole/Zn2+, type of zinc salt reagents, reagent concentrations, addition of surfactants) as well as process parameters (temperature and time) were systematically investigated. Upon characterizations of as-synthesized samples by X-ray powder diffraction, thermal gravimetric analysis, N2 adsorption, and field-emission scanning electron microscope, the results show that the particle size and morphology of ZIF-8 crystals are extremely sensitive to the compotional parameters of reagent concentration and addition of surfactants. The particle size and morphology of hydrothermally synthesized ZIF-8 crystals can be finely tuned; with the size ranging from 90 nm to 4 μm and the shape from truncated cubic to rhombic dodecahedron.

In this study, pervaporation, a membrane-based process was studied for in-situ separation of butanol. This technique has a great potential due to its high selectivity, low energy requirement and high efficiency. The primary objective was to improve the performance of the Polydimethylsiloxane (PDMS) membrane for the pervaporation separation and the recovery of butanol by adding nanoparticles into its matrix to make mixed matrix membrane (MMM). These nanoparticles included zinc-based Metal Organic Frameworks (MOFs) and zinc oxide. Different particle sizes of zeolitic imidazolate framework (ZIF-8) were synthesized. The separation performance of MMMs incorporating different sizes of ZIF-8 nanoparticles was compared to the performance of mixed matrix membranes incorporating zinc oxide as well as pure PDMS membrane. Different characteristics of ZIF-8 and their impact on the performance of the host membrane were discussed. Result showed that the presence of nanoparticles improves the PDMS membrane performance up to a certain particle loading. Moreover, it was shown that the particle size and interfacial bond between polymer and particles have a major impact on the pervaporation membrane separation process. The best membrane for pervaporation separation of butanol from binary aqueous solutions was obtained for the 8 wt% small-size ZIF-8/PDMS MMM where the total permeation flux and butanol selectivity were increased by 350% and 6%, respectively, compared to neat PDMS membranes. In addition to the MOFs, nanotubes are considered emerging nanostructured materials for use in membrane separation applications due to their high molecular diffusivity and unique geometry. Recent progress has also been made on the modification of nanotube surface functionality, and the fabrication of nanotube mixed matrix membranes as well as the ability to align them in MMMs. Since numerous types of nanotubes are available and the process of producing well-aligned nanotube MMMs is very challenging, a theoretical model using finite difference method (FD) was used to gain a deeper understanding on the effect of nanotubes on the separation performance of mixed matrix membranes. A series of numerical simulations were performed and the effects of various structural parameters, including the tubular filler volume fraction, orientation, length-to-diameter aspect ratio, and permeability ratio, were assessed. The results showed that the relative permeability is enhanced by vertically-aligned nanotubes and further increased with an increase of the permeability ratio, filler volume fraction and the length-to-diameter aspect ratio. In addition, comparing the simulation results with existing analytical models for the prediction of the relative permeability acknowledges a need to develop a new correlation that would provide more accurate predictions of the relative permeability of MMMs with embedded nanotube fillers.

Since the discovery of size-selective metal-organic frameworks (MOF) researchers have tried to manufacture them into gas separation membranes. ZIF-8 became the most studied MOF for membrane applications mainly because of its simple synthesis, good chemical and thermal stability, recent commercial availability and attractive pore size. The aim of this work is to develop convenient methods for growing ZIF thin layers on polymer supports to obtain defect-free ZIF membranes with good gas separation properties. We present new approaches for ZIF membranes preparation on polymers. We introduce zinc oxide nanoparticles in the support as a secondary metal source for ZIF-8 growth. Initially the ZnO particles were incorporated into the polymer matrix and later on the surface of the polymer by magnetron sputtering. In both cases, the ZnO facilitated to create more nucleation opportunities and improved the ZIF-8 growth compared to the synthesis without using ZnO. By employing the secondary seeded growth method, we were able to obtain thin (900 nm) ZIF-8 layer with good gas separation performance. Next, we propose a metal-chelating polymer as a suitable support for growing ZIF layers. Defect-free ZIF-8 films with a thickness of 600 nm could be obtained by a contra-diffusion method. ZIF-8 membranes were tested for permeation of hydrogen and hydrocarbons, and one of the highest selectivities reported so far for hydrogen/propane, and propylene/propane was obtained. Another promising method to facilitate the growth of MOFs on polymeric supports is the chemical functionalization of the support surface with functional groups, which can complex metal ions and which can covalently bond the MOF crystals. We functionalized the surface of a common porous polymeric membrane with amine groups, which took part in the reaction to form ZIF-8 nanocrystals. We observed an enhancement in adhesion between the ZIF layer and the support. The effect of parameters of the contra-diffusion experiment (such as temperature lower than room temperature and synthesis times shorter than 1 hour) on ZIF-8 membrane properties was evaluated. We could prepare one of the thinnest (around 200 nm) yet selective ZIF-8 films reported.

No description available.

ZIF-8 is a flexible zeolitic imidazole-based metal−organic framework whose narrow pore apertures swing open by reorientation of imidazolate linkers and expand when probed with guest molecules. This work reports on the crystal size dependency of both structural transitions induced by N2 and Ar adsorption and dynamic adsorption behavior of n-butanol using well-engineered ZIF-8 crystals with identical surface area and micropore volume. It is found that the crystal downsizing of ZIF-8 regulates the structural flexibility in equilibrium adsorption and desorption of N2 and Ar. Adsorption kinetics of n-butanol in ZIF-8 are strongly affected by the crystal size, however, not according to a classical intracrystalline diffusion mechanism. Our results suggest that structural transitions and transport properties are dominated by crystal surface effects. Crystal downsizing increases the importance of such surface barriers.

Mixed matrix membranes (MMMs) have gained importance because they combine the desirable properties of the polymers and the organic/inorganic filler materials and they may have a very big potential. In this study polyethersulfone (PES) was used as polymeric material, and Zeolitic Imidazolate Framework-8 (ZIF-8) was used as porous filler material, and 2-hydroxy 5-methyl aniline(HMA), was used as a third component in membrane formulation. In this study, ZIF-8 crystals were synthesized with varying particle sizes, and a novel recycling methodology was developed to improve the efficiency of ZIF-8 production. ZIF-8 nano-crystals were synthesized by a 1-hour stirring method at room temperature and characterized by X-ray diffractometer, scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS) and thermal gravimetric analysis (TGA). In order to investigate the effect of ZIF-8 loading on the membrane performance, different types of membranes were prepared with varying amounts of ZIF-8 between 10-60% (w/w). Moreover, ternary mixed matrix membranes were synthesized consisting of different amounts of ZIF-8 between 10-30% (w/w) and HMA 1-10% (w/w). Gas transport properties of the membranes were investigated by single gas permeation experiments of H2, CO2 and CH4 at 3 bar feed pressure. In order to investigate the effect of feed pressure on the gas transport properties of the membranes, single gas experiments were conducted on 3, 6, 8, 10 and 12 bar feed pressures. Moreover, binary gas permeation experiments of CO2/CH4 pair were conducted through selected membranes at 3 bar and 12 bar feed pressures. In addition to gas permeation experiments, the morphology and thermal characteristics of the membranes were characterized by SEM, TGA and differential scanning calorimetry (DSC) analysis. The incorporation of ZIF-8 crystals into continuous PES matrix resulted in high performance gas separation membranes. The permeabilities of all studied gases increased with ZIF-8 loading while the ideal selectivities showed a slight decrease compared to neat PES membrane. Highly reproducible and repeatable results were obtained up to 30 % w/w ZIF-8 loading, while membrane formulation reproducibility was decreased for higher ZIF-8 contents (&gt / 30 w/w %). Addition of HMA improved the gas separation performances of the binary membranes significantly by decreasing permeabilities and increasing ideal selectivities. PES/ZIF-8(%20)/HMA(%7) membrane has the best separation performance for all gases among the ternary membranes. When 7 w/w % HMA was added to PES/ZIF-8(%20) membrane, H2 permeability decreased from 26.3 to 13.7 barrer, while H2/CH4 ideal selectivity increased from 61.8 to 103.7. Increasing feed pressures appreciably increased the separation performances of all membranes. While the H2 permeability is pressure independent, the CO2 and CH4 permeabilities were reduced with increasing feed pressures and the highest selectivity improvement was observed in H2/CH4 pair for all membrane compositions. For instance, when the feed pressure was increased from 3 bar to 12 bar, the percentage improvements in ideal selectivities through PES/ZIF-8(%10)/HMA(%4) membrane were calculated as 26, 69, 113 % for the H2/CO2, CO2/CH4 and H2/CH4 gas pairs / respectively. This results show that working at higher feed pressures will be more advantageous for separation of the studied gas pairs. The ideal selectivities and the separation factors were equal to each other for all membrane compositions both for 3 and 12 bar operating pressures.

abstract: This work aimed to characterize and optimize the variables that influence the Gas Diffusion Layer (GDL) preparation using design of experiment (DOE) approach. In the process of GDL preparation, the quantity of carbon support and Teflon were found to have significant influence on the Proton Exchange Membrane Fuel Cell (PEMFC). Characterization methods like surface roughness, wetting characteristics, microstructure surface morphology, pore size distribution, thermal conductivity of GDLs were examined using laser interferometer, Goniometer, SEM, porosimetry and thermal conductivity analyzer respectively. The GDLs were evaluated in single cell PEMFC under various operating conditions of temperature and relative humidity (RH) using air as oxidant. Electrodes were prepared with different PUREBLACK® and poly-tetrafluoroethylene (PTFE) content in the diffusion layer and maintaining catalytic layer with a Pt-loading (0.4 mg cm-2). In the study, a 73.16 wt.% level of PB and 34 wt.% level of PTFE was the optimal compositions for GDL at 70 °C for 70% RH under air atmosphere. For most electrochemical processes the oxygen reduction is very vita reaction. Pt loading in the electrocatalyst contributes towards the total cost of electrochemical devices. Reducing the Pt loading in electrocatalysts with high efficiency is important for the development of fuel cell technologies. To this end, this thesis work reports the approach to lower down the Pt loading in electrocatalyst based on N-doped carbon nanotubes derived from Zeolitic Imidazolate Frameworks (ZIF-67) for oxygen reduction. This electrocatalyst perform with higher electrocatalytic activity and stability for oxygen reduction in fuel cell testing. The electrochemical properties are mainly due to the synergistic effect from N-doped carbon nanotubes derived from ZIF and Pt loading. The strategy with low Pt loading forecasts in emerging highly active and less expensive electrocatalysts in electrochemical energy devices. This thesis focuses on: (i) methods to obtain greater power density by optimizing content of wet-proofing agent (PTFE) and fine-grained, hydrophobic, microporous layer (MPL); (ii) modeling full factorial analysis of PEMFC for evaluation with experimental results and predicting further improvements in performance; (iii) methods to obtain high levels of performance with low Pt loading electrodes based on N-doped carbon nanotubes derived from ZIF-67 and Pt. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2016

Hydrogen is an appealing energy carrier for clean energy use. However, storage of hydrogen is still the main bottleneck for the realization of an energy economy based on hydrogen. Many materials with outstanding properties have been synthesized with the aim to store enough amount of hydrogen under ambient conditions. Such efforts need guidance from material science, which includes predictive theoretical tools. Carbon nanotubes were considered as promising candidates for hydrogen storage applications, but later on it was found to be unable to store enough amounts of hydrogen under ambient conditions. New arrangements of carbon nanotubes were constructed and hydrogen sorption properties were investigated using state-of-the-art simulation methods. The simulations indicate outstanding total hydrogen uptake (up to 19.0 wt.% at 77 K and 5.52wt.% at 300 K), which makes these materials excellent candidates for storage applications. This reopens the carbon route to superior materials for a hydrogen-based economy. Zeolite imidazolate frameworks are subclass of MOFs with an exceptional chemical and thermal stability. The hydrogen adsorption in ZIFs was investigated as a function of network geometry and organic linker exchange. Ab initio calculations performed at the MP2 level to obtain correct interaction energies between hydrogen molecules and the ZIF framework. Subsequently, GCMC simulations are carried out to obtain the hydrogen uptake of ZIFs at different thermodynamic conditions. The best of these materials (ZIF-8) is found to be able to store up to 5 wt.% at 77 K and high pressure. We expected possible improvement of hydrogen capacity of ZIFs by substituting the metal atom (Zn 2+) in the structure by lighter elements such as B or Li. Therefore, we investigated the energy landscape of LiB(IM)4 polymorphs in detail and analyzed their hydrogen storage capacities. The structure with the fau topology was shown to be one of the best materials for hydrogen storage. Its total hydrogen uptake at 77 K and 100 bar amounts to 7.8 wt.% comparable to the total uptake reported of MOF-177 (10 wt.%), which is a benchmark material for high pressure and low temperature H2 adsorption. Covalent organic frameworks are new class of nanoporous materials constructed solely from light elements (C, H, B, and O). The number of adsorption sites as well as the strength of adsorption are essential prerequisites for hydrogen storage in porous materials because they determine the storage capacity and the operational conditions. Currently, to the best of our knowledge, no experimental data are available on the position of preferential H2 adsorption sites in COFs. Molecular dynamics simulations were applied to determine the position of preferential hydrogen sites in COFs. Our results demonstrate that H2 molecule adsorbed at low temperature in seven different adsorption sites in COFs. The calculated adsorption energies are about 3 kJ/mol, comparable to that found for MOF systems. The gravimetric uptake for COF-108 reached 4.17 wt.% at room temperature and 100 bar, which makes this class of materials promising for hydrogen storage applications.